Measurements and calculations of the halfwidth of two rotational transitions of water vapor perturbed by N-2, O-2, and air

In this paper we report the results of both an experimental and theoretical study of the halfwidths of two transitions of water vapor. Measurements on the lines of the (H2O)-O-16 and (H2O)-O-18 isotopomers located at 325.1 an d 203.4 GHz, respectively, were carried out in the temperature range 300-39 3 K, with N-2 and O-2 as perturbing gases. The foreign-broadening coefficie nts and their temperature-dependence parameters were determined assuming a Voigt profile and the usual temperature dependence for the halfwidth. The r etrieved values are compared to values calculated using the complex semicla ssical formalism of Robert and Bonamy. The assumed intermolecular potential is a combination of electrostatic and atom-atom components. This last cont ribution is defined as the sum of pairwise Lennard-Jones 6-12 interactions between the atoms of H2O and the atoms of the perturbing molecules expanded to eighth order. Also calculated are the pressure-induced shifts of the sp ectral Lines for temperatures from 200 to 400 K. Calculated and experimenta l results are in good agreement, within +/-3.2%, except for the N-2-broaden ing temperature coefficients, for which there are discrepancies as high as 23%. Air-broadening parameters are determined following the classical relat ion: gamma (air) = 0.79 gamma (N-2) + 0.21 gamma (O-2). (C) 1999 Academic P ress.